A superheterodyne receiver is currently the most common type of receiver used in modern communications devices. Such receivers can be found in virtually any home, office or automobile within a television set, telephone or radio. A superheterodyne receiver mixes (or multiplies) an incoming radio-frequency (RF) signal (carried at frequency f.sub.1) with a sinusoid signal (at a frequency f.sub.2) generated by a local oscillator. The resulting output signal comprises two frequency-shifted versions of the incoming signal centered at the sum and difference of the combining frequencies (f.sub.1 +f.sub.2 and f.sub.1 -f.sub.2). Typically the highest frequency components (centered at f.sub.1 +f.sub.2) are filtered out using a band pass filter and the output signal only contains the intermediate-frequency (IF) components (centered at f.sub.1 -f.sub.2). This process may be repeated several times in high-performance superheterodyne receivers.
While superheterodyne receivers are widely used, they use expensive and non-integrable RF and IF components such as band pass filters. As a result, superheterodyne receivers are not ideal for applications in small, low cost mobile communication systems such as cellular phones, pagers, cordless phones, and the like.
Alternative receivers, such as the direct conversion receiver (or homodyne receiver), are well-known in the art and potentially offer significant advantages over the superheterodyne receiver. A traditional direct conversion receiver as shown in FIG. 1 directly converts an incoming signal into its baseband in-phase and quadrature components without any intermediate translation into an IF signal. The operation of this traditional direct conversion receiver is simple. An incoming bandpass signal g(t) (which can be mathematically represented by g(t)=g.sub.i (t)cos(2.pi.f.sub.1 t)-g.sub.q (t)sin(2.pi.f.sub.1 t) is received at the RF input and then passed through a preselector filter 1 and a low-noise amplifier (LNA) 2. The preselector filter 1 is simply a band pass filter designed to pass the desired signal g(t) and to reject spurious out-of-band signals. In most applications, the bandwidth of the preselector filter is much greater than the bandwidth of the desired signal. Furthermore, the preselector filter may pass unwanted signals in addition to the desired signal.
After passing through the preselector filter 1, the signal g(t) is split and sent through the two mixers 3, 3'. In the upper mixer 3', the signal g(t) is mixed with a sinusoid tuned to the same frequency as the carrier frequency (e.g., cos(2.pi.f.sub.1 t)). In the lower mixer 3, the signal g(t) is mixed with the same sinusoid as in the upper mixer 3', but with a phase change of .pi./2 (e.g., sin(2.pi.f.sub.1 t)). The mixers 3, 3' produce the in-phase (g.sub.i (t)) and quadrature (g.sub.q (t)) components of the desired signal (g(t)) centered at baseband and at twice the carrier frequency (2f.sub.c). The high frequency components are eliminated by the low pass filters 6, 6', and the in-phase and quadrature signals are finally amplified by the amplifiers 7,7'.
There are several advantages of a direct conversion receiver over the more popular superheterodyne receiver. First, the direct conversion receiver directly converts the incoming signal into its baseband signal directly and eliminates the step of initially translating the RF signal into an IF signal. Thus, all of the intermediate filters, mixers and amplifiers can be omitted and the circuit is simplified. Secondly, with the exception of the preselector filter, the direct conversion receiver employs only low pass filters rather than band pass filters. Normally, it is easier to integrate a low pass filter onto a single chip than a band pass filter. Thus, the direct conversion receiver may be largely constructed on a single integrated circuit, which makes it smaller and less expensive than a superheterodyne receiver.
While there are advantages to direct conversion receivers over superheterodyne receivers, the traditional direct conversion receiver suffers from some disadvantages. One problem with traditional direct conversion receivers is second-order distortion present in the mixer. Second-order distortion is caused by the fact that a mixer is inherently a non-linear device. When an off-channel RF signal is detected along with the desired signal, the non-linearity in the mixers produce the second harmonic of the undesired signal at baseband plus a DC offset. Since the direct conversion receiver also shifts the desired signal to baseband, this second-order distortion produced by the mixer can significantly reduce the performance of the receiver. Moreover, the mixer can operate like a "square law" detector and convert the envelope of a strong interferer to baseband. If the envelope of the interferer is constant in time, then a DC offset appears at baseband. In this case, there are several methods known in the art to suppress this unwanted DC offset. For example, the DC offset may be attenuated by high pass filtering the baseband output of the mixers. While this method is effective to eliminate a DC offset, it is ineffective for distortion due to a non-constant envelope of an interferer. Thus, a need exists for a homodyne receiver which is capable of attenuating distortion caused by either a constant or a non-constant envelope of an interferer.
Another problem with direct conversion receivers is spurious emissions. The main source of spurious emissions in a direct conversion receiver is local oscillator leakage. In an ordinary superheterodyne receiver, the local oscillator leakage to the antenna is attenuated by the first receiver bandpass filter. In a direct conversion receiver, however, the local oscillator frequency lies within the passband of the preselector filter. Thus, local oscillator leakage is not suppressed in the traditional direct conversion receiver.